Atronach

With the Atronach birthsign you don't regain Magicka over time. Instead you receive a 50%
spell absorption to help you regen your mana. You'll notice that when you are hit with a
spell that your mana regens that way, of course potions and welkynd stones help too.

If the atronach isn't your birthsign then this would clasify as a major bug.